Effects of the binding of myosin light chain kinase on the reactivities of calmodulin lysines.

Abstract

The effects of the binding of smooth muscle myosin light chain (MLC) kinase on the microenvironments of different regions of calmodulin (CaM) were investigated by comparing the acylation rate constants of the seven lysine amino groups of free CaM with those of CaM complexed with MLC kinase. Equimolar amounts of CaM and CaM-MLC kinase complex were trace labeled with [3H]acetic anhydride in the presence of phenylalanine as a standard nucleophile. After completion of the reaction, equal amounts of a trace 14C-acetylated CaM sample, together with [14C]acetylphenylalanine, were added to each reaction mixture. The 3H/14C-labeled CaM and acetylphenylalanine were then isolated from each solution. After complete reaction with nonradioactive acetylating reagent, 3H/14C ratios (r) were determined for each epsilon-N-acetyllysine in the two CaM samples. These values were obtained either from isolated peptide fragments containing one lysine or from epsilon-N-acetyl phenylthiohydantoin lysine obtained by Edman degradation of peptide fragments containing two lysines. From the ratios, protection factors (= rfree/rcomplex) were determined as a measure of the perturbation produced by MLC kinase binding. These protection factors were corrected, using the isotope ratios of the internal standard, for differences in the degree of competition for labeling reagent between the two mixtures. In two separate labeling experiments employing different levels of trace labeling, very little change was observed in the reactivities of four lysines on MLC kinase binding (lysines 13, 30, 77, and 94). Small but reproducible decreases (about 2-fold) were observed in the reactivities of lysines 21 and 148, while lysine 75 underwent a major (more then 7-fold) decrease in labeling. In conjunction with previously published data, these results are interpreted as suggesting that the major perturbation in lysine 75 is a direct effect of MLC kinase contact with CaM and that a region in the central helix containing this residue, but not lysine 77, represents or is near the CaM-binding site for MLC kinase. The smaller changes in reactivities at lysines 21 and 148 may reflect a conformational change that occurs in CaM as a result of binding to MLC kinase.